1. Field of the Invention
This invention relates to the control of harmonics in the line voltage and current of supplies to electronically controlled equipment. In particular, it relates to switched reluctance drive systems drawing their power from supplies which have limits on harmonic content.
2. Description of Related Art
Electronically controlled equipment is commonly supplied from an AC supply which has a nominally sinusoidal voltage waveform. However the current drawn from the supply by the equipment is frequently non-sinusoidal, due to the nonlinearities within the equipment. The current is generally represented mathematically as a series of sinusoids of different frequencies: the lowest frequency (the fundamental) corresponds to the frequency of the supply and the higher frequencies are known as harmonics. Mathematical tools such as Fourier transforms are routinely used to determine the frequency and magnitude of these components of the current.
In recent years, the electricity supply companies have become increasingly concerned at the increase in harmonic content of the line current supplied to electronically controlled equipment. Regulations have been implemented to limit the amount of permitted harmonics. For example, in Europe the relevant standard for domestic and light industrial equipment is IEC 1000. FIG. 1 shows the IEC 1000 limits of harmonic content for a domestic appliance operating from a 230V single-phase supply and drawing up to 16A input current. For low-power equipment, say up to 600W as found in a small microwave oven, the conventional practice is to fit a passive filter in the form of a simple line choke (i.e. an inductor) in series with the equipment to suppress the harmonics. This choke presents an increasing impedance to harmonics as the harmonic order rises and is generally designed to be just sufficient to allow the equipment to stay within the permitted levels. However, as the power drawn by the equipment rises, the size and cost of these filters becomes uneconomic and some form of active filtering becomes necessary. Typical active filters for domestic appliances are well known and are discussed in, e.g., "UC3854 Controlled Power Factor Correction Circuit Design", Todd, P.C., Unitrode Application Note U-134, Unitrode Corporation, Merrimack, N.H., USA, which is incorporated herein by reference.
Switched reluctance machines are increasingly being used in domestic appliances and other relatively low-power applications. The characteristics and operation of switched reluctance machines are well known in the art and are described in, for example, "The characteristics, design and application of switched reluctance motors and drives" by Stephenson and Blake, PCIM '93, Nurnberg, Jun. 21-24 1993 and incorporated herein by reference. FIG. 2 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The drive is supplied from a single-phase AC mains supply, shown in FIG. 2 as a voltage source 32 in series with a source impedance 34. In most cases, the impedance is mainly inductive, and this inductance can be increased by adding inductance in series, as described above. The rectifier bridge 36 rectifies the sinusoidal voltage of the source and the output voltage is smoothed by the capacitor 38. The lines marked +V and -V are generally known as the DC link, and capacitor 38 as the DC link capacitor. In the absence of any load on the DC link, the capacitor 38 will charge up on successive cycles of voltage to the peak voltage of the rectifier output. The DC voltage provided by the DC link is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The switched reluctance drive is essentially a variable speed system and is characterized by voltages and currents in the phase windings of the machine which are quite different from those found in traditional types of machines. FIGS. 3(a)-3(c) illustrate this point. FIG. 3(a) shows the voltage waveform typically applied by the controller to the phase winding. At a predetermined rotor angle, the voltage is applied by switching on the switches in the power converter 13 and applying constant voltage for a given angle T.sub.c, the conduction angle. The current rises from zero, typically reaches a peak and falls slightly as shown in FIG. 3(b). When T.sub.c, has been traversed, the switches are opened and the action of energy return diodes places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then a period of zero current until the cycle is repeated. It will be clear that the phase is drawing energy from the DC link during T.sub.c and returning a smaller amount thereafter. It follows that the DC link needs to be a low-impedance source which is capable of receiving returned energy for part of its operating cycle. FIG. 3(c) shows the current which has to be supplied to the phase winding by the DC link and the current which flows back during the period of energy return.
The size of the DC link capacitor 38 and the amount of current drawn by the drive clearly interact, and general practice is to size the capacitor so that there is a relatively small amount of droop on the DC link voltage while the capacitor is supplying the load during the periods when the AC supply voltage has fallen below the capacitor voltage. This ensures that the load is operated from an essentially constant voltage. When of an appropriate size, the capacitor is able to supply most of the required higher-order harmonic currents, but gives rise to large amounts of lower-order (i.e. 3.sup.rd and 5.sup.th) harmonics unless additional filtering is used. This arrangement also has the disadvantage of requiring a large, and therefore costly, capacitor. To mitigate these problems, it is known to use a DC link arrangement which is both economic in capacitor size and has an improved power factor. FIG. 4 shows one such circuit, the "valley-fill" circuit, which can meet both of these requirements. However such circuits are generally poor at supplying the higher order harmonic currents from the capacitors and source these harmonics from the mains supply.
FIG. 6 shows measurements of the conventionally operated machine running at top speed. The upper two traces show the gate firing signals applied to the two switches of one phase. As will readily be understood by those skilled in the art, the dc bus voltage is applied to the phase winding when both the gate firing signals are high. In this case, the single excitation pulse is shown as occupying around 50% of the cycle time and is followed by a brief period of freewheeling. Because of the valley-fill circuit, the DC link voltage fluctuates, and the particular energization cycle shown corresponds to an instant when the voltage is high, resulting in the machine operating in the continuous current mode, as described in more detail in, e.g., U.S. Pat. No. 5,469,039, incorporated herein by reference. FIG. 7 shows how the two phase currents combine to give the supply current. The figure shows approximately one half cycle of the alternating supply current. It will be realized that the supply current is zero when, by the action of the valley fill circuit, the capacitors supply all the energy to the load. This supply current has a large harmonic content, as shown by its analysis in FIG. 8. This analysis can be done by a number of known methods, commonly using a Fourier transformation implemented by a standard piece of test equipment. FIG. 8 also shows the limits of the harmonics as detailed in FIG. 1, from which it is readily seen that the drive exceeds the permissible limits for the measurements marked with an asterisk (*).
There is a need for a system which has high power factor, draws low harmonic current from the supply and is economical to produce.